Getting Started with OpenRocket

Below are several videos detailing the features of OpenRocket and how to use them. More video tutorials may be added as needed.

Basic Rocket Design

Installing Motors and Running Simulations

Tubing

Tubing is probably the essential airframe component as it makes up almost all of the exterior structure and shape of the rocket. Historically, we have mainly used BlueTube as our default tubing material, but we are moving towards carbon fiber for our larger rocket designs as it offers a great combination of strength and low weight.

Payload Tube

The Payload Tube is the tube dedicated to housing the payload, whatever it may be. This is generally directly under the nose cone as the payload is often partially stored in the nose cone as well to efficiently utilize all available space.

Avionics Bay (Av Bay)

The Avionics Bay (or Av Bay) houses all of the electrical boards, flight computers, and avionics of the rocket. As this is a very delicate section of the rocket, it is generally closed/sealed on both ends by bulkheads. It also usually has a door, sled, or other form of access so the Avionics team can access the boards at anytime time, even when the rocket is on the launch rail.

Recovery Tube

The Recovery Tube houses the parachutes (and supporting recovery components) of the rocket. This section of the rocket has to be able to separate to allow the parachutes to release after apogee has been reached. In the past this separation has been done via black powder.

Booster Tube

The Booster Tube is at the bottom of the rocket and houses the motor. It is generally sealed off from the rest of the rocket.

Couplers

Couplers are tubes that work as connecting sections of the rocket that have a slightly smaller diameter than the rocket itself, so that they can fit snugly inside of it and allow different rocket tube sections to mate. They are permanently attached to these tube sections and generally made out of the same material as the main tubing.

Bulkheads

Bulkheads are the "dividing walls" of the rocket or in other words structural sealing tools that are fitted inside the tube and comprise the entire area of the inner tube. They are used to seal off sections of the rocket where we do not want any interaction, such as between the motor and whatever is above it. They are also used as structural mounting spots for things like parachute u-bolts. Sometimes they have holes so that pipes can pass through them. Historically, we have made these out of wood. We are planning to use acrylic for our larger rockets.

Centering Rings

Centering rings are structural tools used to hold things in place inside of the rocket. They are similar to bulkheads except that they have a hollow center (ring instead of circle). We have used them to secure the payload in the nose cone and secure the motor tube inside of the booster tube. When used to center the motor, they should be strong enough to withstand high impulses that the motor produces during flight. Historically, we have made these out of wood. We are planning to use aluminum for our larger rockets.

Nose Cone

The cone shaped nose of the rocket that is designed to reduce drag at the front end/top of the rocket. We generally go for nose cones that are made of carbon fiber and have a 4:1 length to diameter ratio (a 6in diameter rocket would have a 24 in length nose cone).

Transition Piece

A custom piece of tubing that is made to facilitate a diameter transition in a rocket. For example, a transition piece was used in Arktos to transition from a 6in diameter (nose cone and payload tube) to a 4in diameter (recovery, booster, av bay). Transition pieces allow for versatility by allowing certain parts of the rocket to house larger diameters without requiring the entire rocket to commit to the larger size.

Fins

A stabilizing agent that is fitted to the bottom of the rocket.

Tail Cone

Similar to the nose cone, but at the very end of the rocket. A tail cone exists to buffer the change at the bottom of the rocket from "whatever diameter" to nothing (i.e. where the rocket ends). Adding in a piece that gives a gradual change in diameter helps to eliminate drag and achieve a higher apogee.

Spray Paint

Rustoleum 2-in-1 Paint+Primer has worked fine in the past; generally people use spray paint to paint rockets.

1. Prepare surfaces for painting. For fiberglass parts, this means clean with isopropyl alcohol / water mixture, sand lightly with 100+ grit sandpaper, and then clean off dust with tack cloth or more IPA mixture.

2. Apply a light coat of "primer" (may also be paint+primer). No need to use the same color that your final coat will be, but choose a light primer color if you want a light-colored part.

3. Apply one to two more light coats of primer, waiting about a minute in between each coat. Do not worry about completely covering all spots, but do your best to apply a thin, even coat. Follow the instructions on the can with respect to distance from the part.

4. Wait the required amount of time (usually 24 hours) for the base coat to dry

5. Apply 2-3 coats of the final color you want, about one minute apart. Do your best to avoid spending too long on one spot; it's easy to apply another coat, but it's hard to undo a puddle or run!

6. Wait 24-48 hours for the outer coat to dry

7. Apply 2-3 light coats of clear coat, moving slightly more slowly on the last coat to achieve a glossy finish. The clear coat will protect the paint underneath.

Safety

• Always wear gloves when handling epoxy and composite fibers.

• Keep hands, feet, hair, etc. out of the way of the X-Winder when in operation. It is a large machine and there is a good chance it could cause as much damage to you as it will to itself.

• When using ovens, avoid accessing them while hot, and wear necessary safety equipment when handling hot objects.

Setup

1. Select the desired tow spool and epoxy. The epoxy that is used should have a setting time that is longer than the estimated wind time to avoid issues with X-Winder operation.

2. Ensure the X-Winder is clean and in working order: motors turn smoothly without overheating, tow spool rotates freely, belts are secured tightly, no residual epoxy, etc.

3. Mount the desired mandrel to the main rod, making sure that everything is tight and does not slip when rotated.

4. Make accurate measurements of the tow line and the mandrel, as well as the start and end lengths of the wind pattern. (Note that generally the ends of a wind are inconsistent with the bulk, so it is best to wind 2-3 in. longer than the desired tube length and then cut to size after.)

5. Input these measurements into the X-Winder software along with desired wind angles and layer count.

6. Cut a piece of bleeder/breather cloth that will fit around the mandrel for use after the wet wind.

7. Depending on the estimated wind time, this is a good point to think about pre-heating the curing oven to the desired temperature.

Dry Wind

1. Pull the tow through the rollers to the delivery head. Be sure to place the line between all spacers and check that no fraying occurs as the tow is pulled through.

2. Tie the end of the tow to the mandrel slightly ahead of the start location such that the knot is not wound over. This knot has to be secure since there will be significant tension as the wind starts. Tape can be used to help secure the knot.

3. Run the software for part of a layer, checking for proper spacing of the wind and wind behavior at the ends of each pass. It is a good idea to check each different wind angle and verify that things look good and the X-Winder is working as intended.

4. If everything is good to go, cut the tow and unwrap the partial wind. Remove the mandrel to prepare it for a wet wind.

Wet Wind

1. Wrap the mandrel in wax paper such that the paper is snug to the surface but can still slide off without too much effort. The wax paper should be longer than the intended wind length but shorter than the mandrel. Tape the ends of the paper to the mandrel so that it cannot rotate independently during winding.

2. Apply several thin coats of mold release agent to the surface of the wax paper, allowing 5-10 minutes to fully dry.

3. Place mandrel back on the X-Winder, making sure that starting and ending wind measurements are within the wax paper region.

4. Mix one pump of resin and hardener and pour into the epoxy tray. Pull the tow through the tray until epoxy has reached the delivery head. Ensure that the epoxy regulator is properly tensioned.

5. Tie the end of the tow to the mandrel slightly ahead of the start location such that the knot is not wound over. This knot has to be secure since there will be significant tension as the wind starts. Tape can be used to help secure the knot.

6. Run the wind program. Watch to make sure things are running smoothly and that the first few passes are looking good.

7. Every 10-15 minutes, check the epoxy tray to make sure there is enough to cover the bottom part of the tray. Do not overfill the epoxy tray, as this will cause a buildup of heat as the polymerization reaction occurs, which can melt the tray or cause curing inconsistencies.

8. It is good practice to pause after each layer just to give everything a quick inspection before proceeding.

9. At the end of the wind, cut the tow and take a moment to appreciate the fact that the hard part is over!

Finishing and Curing

1. Wrap the wind in one layer of bleeder/breather cloth. Try not to overlap too much as it might dry out the surface.

2. Tape and secure one end of a roll of shrink tape just off of the end of the wind. Run the shrink tape program or simply have the software spin the mandrel as you slowly wrap the shrink tape around the wind. Be careful: try to avoid wrinkling the tape and keep an even overlap as you move across the wind. Cut and secure the other end once the wind is completely wrapped.

3. Remove the mandrel and rod from the X-Winder and place in the curing oven. (The temperature should be above the activation temperature of the shrink tape.)

4. After the wind has been completely cured, cooled, and removed from the oven, the shrink tape and bleeder/breather cloth can be removed.

5. Remove the tape holding the wax paper to the mandrel, then remove the composite tube from the mandrel. Peel away the wax paper from the inside of the tube.

6. Congratulations! You have produced a composite filament wound tube. Inspect it for any defects or flaws and appreciate its cool pattern. It is now ready to be cut, sanded, turned into a rocket!

Clean-Up

1. Disassemble the parts of the X-Winder which touched epoxy. Thoroughly clean these using a solvent such as acetone.

2. Check for and remove any fraying residue and clumps in and around the area.

3. Discard any epoxy mixing cups, used shrink tape, used bleeder/breather cloth, excess tow, and all other waste.

4. Make sure the X-Winder is unplugged when not in use.

Terminology

• Root chord - edge of fin attached to body tube

• Tip chord - edge of fin parallel and furthest from body tube

• Leading edge - the edge facing the front

• Trailing edge - the edge facing the rear

• Semi-span - distance from the root to tip chord

• Aspect ratio - ratio of a fin’s span squared to its area

• Taper ratio - ratio of tip to root chord lengths

Fin Sizing

• Root chord: ~2 diameter lengths

• Tip chord: ~ 1 diameter length

• Semi span: vary this dimension for appropriate stability

• Fin tabs: make contact with the motor tube and typically between two centering rings.

• Placement: close to the back of the rocket between two centering rings.

• Material: The main options for the fin material are plywood, fiberglass, and carbon fiber. The material depends on the rocket being made and the durability needed.

• Fillets: Create fillets between the fins and the airframe using epoxy. This will increase aerodynamics while ensuring the fins are reinforced.

• Sanding edges: Sand the leading edge and tip chord of the fins to decrease air resistance and increase aerodynamics. This is optional, but highly recommended.

• Check the Airframe OpenRocket tutorial to learn about adding and designing fins in OpenRocket.

Fin Parameters

Fin Flutter

As the rocket flies at high speeds, the fins will vibrate. For lower speeds, this is not a problem because the amplitude of vibrations will decrease from the air. This is problematic when the rocket speed exceeds the maximum fin flutter speed at which point the air will amplify oscillations to the point of destroying the fin. The maximum fin flutter can be calculated from the following formula:

• Flutter speed (Vf) - max speed before the fins break

• Shear Modulus (G) - amount of deformation associated with a certain amount of force

• Speed of Sound (a)

• Wing Thickness (t)

• Root Chord (cr)

• Tip Chord (ct)

• Semi Span (b)

• Air Pressure (P)

It is important to dimension your fins so their maximum fin flutter lies above the maximum rocket speed.

Fin Thickness

Thicker fins are more structurally stable, but they also increase the weight of the rocket and the drag experienced during flight. The force of drag can be calculated with:

• Drag Force (Fd)

• Air Pressure (p)

• Velocity (v)

• Drag Coefficient (cd) - how well air moves around the fins

• Area (A) - increases with more thickness

The drag coefficient can be lowered by improving the cross sectional area of the fin. Cross sectional areas include square, rounded, and airfoil in the order of lowest to highest performance. The fin thickness should also account for fin flutter as a low thickness can risk damaging fins during flight.

Stability

The primary purpose of fins is to correct the rocket during flight such that it continues on a stable trajectory. In order to do this, the center of pressure should lie below the center of gravity. This is so the rocket is stabilized or pointed upward if there is a deviation from the stable configuration. The center of pressure is the sum of the pressure field on the rocket, which creates a lift force.

• Stability (S) - measured in cals

• Center of Pressure (CP) from the front of the rocket

• Center of Gravity (CG) also from the front of the rocket

• Rocket diameter (d)

As a general rule of thumb, the stability should fall between 1-2 cals. Below this range, the rocket may not correct itself enough. Above this range, the rocket may overcorrect. By increasing the surface area of the fins, the center of pressure will move towards the aft end and increase the stability.

Fin Jig

Fins can be attached with a fin jig. This method involves epoxying the fins onto the motor mount, at equal spacing, through slits made on the main booster tube. We ensure that the fins stay perpendicular to the airframe by using a fin jig: a lasercut "spacer" that holds the fins in place while the epoxy dries.

Fin jigs are used for fin sizes where epoxy adhesion is sufficient. For larger or heavier fins (here we used fiberglass), it might be best to use fin brackets.

To be added later: schematic of fin jig (emphasis on the hole for the rail button), epoxy used, carbon fiber fillets, circle clamp, sanding fillets.

Fin Brackets

As stated above, fin brackets are convenient for larger and heavier fins where epoxy is not strong enough. A fin bracket is typically an L-bracket that gets bolted into the side of the fin and the airframe. We have not had to use fin brackets yet.

Fin Can

A fin can is a single-piece setup that includes all the fins attached to a cylinder that slides onto the booster tube. We have also never used this method.

Tip-to-tip Layups

To quote:

High performance rockets put a huge amount of stress on the fins. Large heavy rockets put large amounts of torque on the fins and high speed rockets can cause the dreaded fin flutter. All large rockets subject fins to high forces on landing.

Reinforcing wooden fins with fiberglass or other composite reinforcement helps to make them stronger. (G-10 fins generally don't need reinforcement for strength.) However, for very high speed rockets, you also need to stiffen fins and carbon fiber makes an excellent reinforcement for this purpose.

Fins can be covered with appropriate reinforcement before being mounted to the body. This will make the fins stronger and stiffer. For conventional rockets with motor mount tubes smaller than the body tube, the fins are bonded at three points: outside the MMT, inside the BT and outside the BT. However, for minimum diameter rockets, the fins are bonded only at one point: outside the BT.

For minimum diameter rockets, it is desirable to reinforce the fin/BT joint for strength. In addition, because minimum diameter rockets are often high performance, it is desirable to stiffen the fins as well. The best way to do this is to laminate the fins tip-to-tip with carbon fiber and fiberglass. By laminating the fins tip-to-tip (and over the body tube in between), we reinforce the joint, stiffen the fin and make a solid fin can.

Separable Interfaces

Friction

Rocketeers traditionally use friction fits for low-power, mid-power, and most L1 and some L2-level rockets. Take this example of a rocket with a single-deploy, motor ejection recovery system:

There are three interfaces marked with vertical lines; the green one is the only interface required to be separable, as it is where parachutes exit the vehicle. In this case, we rely on the friction between the electronics bay coupler (fore) and the booster tube (aft) to keep the upper and lower section from moving relative to each other on ascent after the motor has burned out.

Drag Separation

If a friction-fit interface is not tight enough, drag separation can occur. While separation during powered ascent is less likely, after the motor has stopped producing thrust, it is possible that the drag force experienced by the lower section of the rocket (including fins) is greater than that experienced by the upper section. When this imbalance of forces occurs, it is possible for the lower section to accelerate relative to the upper section. This is known as drag separation, and is not always a bad thing; it can even be desirable if used for stage separation.

A Practical Guide to Friction Fits

It can be more of an art than a science to get a good friction fit. Generally, we recommend following this (paraphrased) advice from Dave Raimondi (ex-LUNAR President, L3-certified):

Your friction fit should allow you to gently lift the rocket in the air by the upper section and hold it such that it is stable and not touching the ground. Then shake the rocket and make sure the bottom section separates with some effort, but does not require violent shaking.

To adjust your friction fit, either: remove/add masking tape to the coupler, or, if no masking tape remains and it is still too tight, sand down the coupler/inside of body tube. We recommend adding tape one layer at a time, either in entire rings or even half rings for fine-tuning. Use wide painters or masking tape for best results (> 1" wide). There is a fair amount of tolerance on the above advice; don't be too worried if your fit seems to be a little too loose or a little too tight.

When Not to Use Friction

Any rocket with dual-side dual-deploy recovery will require a stronger interface to keep the main parachute from coming out. Also consider using a stronger interface for larger and heavier rockets, as they may be subject to larger forces. Refer also to the many forum threads like these for more information:

Shear Pins

Shear pins are fasteners designed to hold an interface together, but break (shear) when recovery energetics (black powder, usually) are activated. They may also be used to retain deployable payloads. The driving mechanism for shear pin failure is the transverse loads applied by each section of tubing (coupler, body tube) as pressure is built up inside the airframe; the shear pins are not vaporized, melted, or otherwise affected by recovery charges.

Shear Pins Selection

STAR members have traditionally used #2-56 or #4-40 nylon screws (e.g., from McMaster-Carr) as "shear pins". While these screws technically have threads, they are often more of a press-fit than screwed into the airframe. No female threads (nut/threaded insert) are required. Shear pins have been effectively used with BlueTube and fiberglass airframes.

Shear Pin Sizing

Using too many or too few shear pins can result in extreme quantities of black powder being required, or the premature separation of the airframe in flight, respectively. STAR has experience with both of these scenarios. Only testing can truly help you avoid these outcomes. Short of testing, precise calculation of the loads may be helpful; however, it is generally quite difficult to estimate exactly what loads will be applied to each shear pin.

Note that dynamic loading when the main parachute opens is usually far higher than any other load during flight; if shear pins are used to retain a payload through/after main parachute deployment, pay special attention to this interface to ensure it does not break prematurely.

Non-Separable Interfaces

When it comes to larger or more complex rockets, it is expected that you will have one or more interfaces that you need to be separable during assembly, but do not come apart during flight. These are generally held together with some sort of fastener. One common example of this type of interface is a nosecone that detaches from the payload tube to allow for the insertion of a payload, but does not need to detach during flight.

Standard Nuts

It is certainly possible to epoxy an ordinary hex nut (see: Fasteners) to the inside of a coupler and thread into it with a machine screw. That being said, we recommend using one of the below options for better reliability and/or convenience. Trying to properly position a normal or low-profile hex nut can be difficult and can result in getting epoxy in the threads or a poor bond with the airframe.

Nut Plates / Weld Nuts

Nut plates and weld nuts essentially refer to the same thing: a normal nut, but attached to a wide base that permanently attaches to a surface. Once the weld nut is attached to a surface, it offers female threads for a removable but secure attachment (similar to a threaded insert). Traditionally in aerospace (especially planes!), nut plates are attached to a surface with rivets while weld nuts (more common in cars) are literally welded to a surface. "Adhesive-mount nuts" are also sold with the explicit purpose of being attached with an adhesive, although most weld nuts/ nut plates are fine to use with epoxy.

To use a weld nut or nut plate with epoxy for a coupler-body tube interface, follow these rough steps (also see below for references with pictures):

1. Test fit coupler and body tube together and tape/ hold interface so tubes do not rotate relative to each other

2. Drill a hole (free fit tolerance for the screw that will be used) radially through both body tube and coupler

3. Insert a screw radially inward through the hole, going through both the body tube and coupler.

4. Hold the nut on the inside of the coupler and thread it onto the screw

5. Mark out area for epoxy around footprint of nut

6. Remove nut and apply epoxy, taking care to avoid the hole where the screw will go. Remember that when the epoxy is compressed, it will spread out, but should not enter the screw/nut interface.

7. Thread nut back on to screw, stopping right before it touches the epoxy

8. Pull screw radially outward, pressing nut into epoxy

9. Hold nut static (use pliers if needed, clean afterward with isopropyl alcohol) while screwing in screw completely to apply medium pressure

10. As epoxy cures, make sure that the screw is still removable. It is very possible to accidentally permanently epoxy the screw to the nut, rendering the connection useless. We recommend keeping pressure at least until the epoxy has set, periodically removing the screw to check that the threads are still useable

Nut Plate References

See this fantastic tutorial on how to use weld nuts/nut plates with fiberglass airframes: http://hararocketry.org/hara/how-to-use-weld-nut-plates-on-fiberglass-rocket/ A similar write-up can be found in Apogee Newsletter 341: https://www.apogeerockets.com/education/downloads/Newsletter341.pdf

Sheet Metal Screws

Historically, STAR has used #4-40 pan head sheet metal screws (from ACE Hardware or McMaster-Carr) to semi-permanently attach Blue Tube interfaces. Sheet metal screws are similar to wood screws in that they have deep, aggressive threads and a sharp point; however, unlike wood screws, they are threaded all the way until the head. This property makes them useful even at very short lengths (1/2" or 1/4" long).

Limits on Sheet Metal Screws

As a sheet metal screw directly cuts into the airframe, the material that said sheet metal screw is holding onto is gradually removed each time the screw is inserted and removed. Practically, this manifests itself as the screw feeling loose and/or simply falling out after too many uses. The screw may also bind in the interface at an angle, instead of remaining perpendicular to the long axis of the rocket.

While Blue Tube generally accepts ~10 or more assembly/removal cycles without any issues and up to 20-25 without serious concern, you may start to notice sheet metal screws in fiberglass becoming loose after as few as 4 cycles (typ. 6-8). This is in part due to the fact that Blue Tube, as a paper composite, will recover its shape more easily after being deformed. While it is possible to attempt to remedy a too-large hole with some epoxy, it is often easier to simply drill another hole and fill the previous one entirely. Depending on the epoxy used, this may take up to 24 hours to completely cure. For a project team on tight assembly timelines and an interest in professionalism and reliability, we do not recommend sheet metal screws for composite airframes. Do not underestimate the potential timeline and build quality impact a poor tube connection can cause.

Self Clinching Nuts (AKA PEM nuts)

Self clinching nuts, sometimes called PEM nuts or press fit nuts, are nuts designed for installing a permeant fixture of female threads in a hole of sheet metal.

After a hole is drilled with the right diameter, the nut can be press fit into the hole. This process will deform the metal to envelope the back tapered shank and hold the nut in place, as well as imbed serration to provide torque resistance.

Rockets are usually not made of sheet metal, but these nut have been seen to work on fiberglass tubes. Do note that for tubes under 2.5" in diameter, the curvature of the tube may be too great for the nut to properly work, as they are design for flat surfaces. It is also important to buy nuts that are suited for the thickness of the tube wall. Additionally, ensure you have the right size drill bit, as hole diameter is crucial to ensure the nut press fits well.

Specialized tools can be used to press fit the nuts into place, but simpler methods can also be effective. By using a screw or bolt that is compatible with the nut, one can tighten the screw and effectively "press" the nut into the drilled hole. A washer can be used to create a better clamping surface, but may not be necessary.

Some people choose to also add epoxy to the nut to increase the strength of the nut to the tube. It likely depends on serval factors for how well the nut actually stay in place, but in flight when the nuts are engaged with the screws, they shouldn't go anywhere. The screw shearing off is more likely than the nut failing all together. Multiple nuts should be used to make a good permanent connection between to pieces of the rocket. It is also recommended that the screw sizes should be slightly longer than they need to be, so in the case of the screws shearing off, they can still be removed from the nuts. Even then, it is recommended to not use these nut for shear pins/screws, and to go with the more traditional technique outlined above.